2017 27RR, first weekend out, first issue. Why does the fresh water drain have a T off it that runs a line back up to the screened vent next to the fresh water gravity fill? The overflow/vent tube is 10 inches long and goes straight down. I'm not a hydrologist, but it would seem that the overflow/vent tube should be the one that runs 6 feet back up to that vent. Why in the world would the drain line need to be vented?

Second post was a pic of the drain line. I must say that I didn't trace that line all the was back up, it gets lost in the insulation, so your probable correct that its the pump feed line. It parallel's the fill tube. The over fill tube just hangs down next to the tank about 8 inches. I mostly dry camp in the forests, so max water is important to me. I'm thinking I will install a ball valve that I'll leave open during filling, then close it for traveling so the tank doesn't slosh half the water out. I don't know if anything is connected to that screened vent at the fill port.

Second post was a pic of the drain line. I must say that I didn't trace that line all the was back up, it gets lost in the insulation, so your probable correct that its the pump feed line. It parallel's the fill tube. The over fill tube just hangs down next to the tank about 8 inches. I mostly dry camp in the forests, so max water is important to me. I'm thinking I will install a ball valve that I'll leave open during filling, then close it for traveling so the tank doesn't slosh half the water out. I don't know if anything is connected to that screened vent at the fill port.

Yes, it's very possible the vent has come loose from the fill port and has fallen down. That would explain why you're losing 1/2 your water.

Either putting back on the fill port or installing a ball valve should fix the loss of water problem.

Talked to mechanic and was told that manufacturers aren't running the vent tube back up to the vent, saves time and money. So it's practice now that the vent tube is just an 8 inch run next to the tank!?!? I'm going to install a ball valve in that short vent line so I can close it while traveling to the woods stopping my fresh water from sloshing out while driving. I'll open it once parked to allow the tank to breath during pump operation.